Light made simple, part 3, lumen maintenance

Lumen maintenance, something with a lot of interpretations and incorrect or incomplete use in specifications. What is it and how to read the numbers. This blog will be about this issue. Let’s start with the different issues that come into play and finally how this should be specified on datasheets.

Most talked about is the first fraction, Lx. This factor on its own does not really explain much. First of all it gives the x, this is specified as a percentage. Lets say that it says 70 it means that is is about a lumen maintenance of 70%. So what does it mean? The number that is mentioned gives the time after which the light output of the product can be degraded to 70% of the lumen compared to the starting value.

That is not enough because it would suggest that all products will be having this light output but in-fact also LED products will fail and that has to be factored in. The factor By has to be taken into account. The By factor is gradual degrading of the lumen output. So sudden failures are not included. The factor LxBy for example; L70B50 means that 50% of the products will have a lumen output of 70% or more of the starting value at the given time. Normal values of B are 50 or 10 meaning 50% is less or 10% is less than the given lumen maintenance.

You probably guessed it but there is also a factor for sudden failures; i.e. LxCy. L being in this case always 0 because it is about complete failure. Cy should be 10 or 50. Again the same, the time indicated gives you information on the percentage that might fail before the given time. So 10% (or less) of the population will have a catastrophic failure before the end of the time.

Than we need the combination LxFy factor. Indeed this is the one you should be looking for. This gives the combination between all of the factors above. L should be specified as 70, 80 or 90%. The F factor should be 10 or 50%. L70F10 means that at the time indicated 90% of the products will still emit at least 70% of the lumen output at starting. Or in other words 10% of the products might have degraded below 70% of the initial lumen output.

Now in the market we see several different indicators and in many times they are not complete. It is however important to check what the factors are to be able to make an informed decision. Lets say that the data sheet stated L85 of 100.000 hours. What does it mean? It says not to much since the failure fraction is missing. So this factor can be true but maybe at a failure fraction of 50%. It can not be read from this information.

Is this important? Maybe or maybe not. If the supplier sufficiently warrants the products he will have to replace the defective products. In his calculations he will factor in the failures. This will be part of the package but are you 100% sure that the manufacturer is still in business after the 100.000 hours for which a warrantee was agreed?

If you are sure about it you can take the risk but if you are not sure, you might want to make more calculations taking into account the failure fraction as well.

On several product data sheets I have seen a lumen maintenance factor. The failure fraction is missing in several cases, so it says nothing.

Than you have the issue of how the data was obtained. What we see is that luminaire manufacturers are solely basing there factors on LM80 data of the manufacturer of the LED’s. A luminaire however almost always contains a driver. This driver is also a piece of electronics that can be failing.

Also the conditions under which the LED is used influences the actual life expectancy. It is not enough to have LM80 data of the LED’s. It has to be checked on actual conditions of use and the driver must be taken into account in the actual failure fractions.

The standard gives some preferred numbers but currently manufacturers use what suites them best. Maybe that will change in the near future, the standards are evolving and IEC PAS that describes this issue is still recent.

Conclusion; check the data sheets clearly and look for the LxFy factor. This will at least give an idea on the performance of the product.

Definitions:
– Lx Lumen maintenance factor, percentage of remaining lumen output after a given time of use.
– By Failure fraction, percentage of products that might fail by gradual degrading of the lumen output below the given percentage (Lfactor) after a given time of usage.
– Cy Catastrophic failure fraction, percentage of products that might fail catastrophic, where the lumen output has decreased to 0, after a given time.
– Fy Failure fraction where Catastrophic and Degrading failures have been combined in one factor. The factor denotes the number of products that might have failed,being catastrophic or by slowly degrading, below the given percentage (Lfactor) after a given time of usage.